This invention relates to composites based on thermosetting cyanate esters, and more particularly to fiber-reinforced composites wherein the matrix resin comprises a cyanate ester and a urea compound latent cure acceleration.
Cyanate esters are well-known in the art and widely used in formulating adhesives, binders, coatings and impregnants. Such formulations may also include oligomeric compounds with reactive cyanate ester functionality as well as a variety of other coreactants such as epoxy resins in order to reduce costs and to modify properties such as toughness, moisture sensitivity and thermal behavior in the resulting thermoset materials.
Cyanate esters may generally be cured merely by heating. Catalysts used to promote curing under milder conditions have included Lewis acids such as aluminum chloride, ferric chloride and the like, mineral acids such as hydrochloric acid, salts such as sodium acetate, potassium thiocyanate and the like, phenolic compounds and basis such as sodium methoxide, pyridine, triethylamine and the like. Metal chelates such as copper, zinc or ferric acetylacetonates have been reported as being capable of promoting a smooth, controllable cure rate at moderate temperatures. Such catalysts are said to be generally less moisture sensitive, and possibly less hazardous than many of the catalyst systems available for cyanate esters.
Many of the prior art catalysts are highly active and many even promote rapid curing at room temperature in many cyanate ester formulations. The storage stability of cyanate ester materials and formulations based on such catalyst may therefore be brief, making the formulations difficult to use for many applications by requiring storage conditions that may be difficult or impractical to achieve. The more stable cyanate ester formulations, those based on the less-active prior art catalysts, may be more difficult to cure adequately even when extended cure cycles are used. Extended cure times, particularly at evaluated temperatures, increase the cost of production and may cause damage to substrates as well as to other components of the formulation. In addition, insufficient cure levels tend to result in brittle materials having an increased sensitivity toward moisture. Cyanate ester cure accelerators having little or no catalyst activity at or near room temperature and a high degree of activity at moderately elevated temperatures are thus needed. Such accelerators are termed latent cure accelerators, and may be used to provide storage-stable cyanate ester formulations that are rapidly and completely cured at moderate temperatures.
Some of the presently available catalysts exhibit a degree of latent curing character or latency when used in combination with some cyanate resins. However, such catalysts are few in number. The uses of cyanate ester formulations by the coatings, adhesives and laminating arts encompass a great variety of applications. The curing conditions required by these applications will vary widely, and the latent curing behavior needed for some applications may be measured in hours, while others may require stability for days or even weeks at room temperature. Moreover, the residues characteristic of some catalysts may not be acceptable for particular applications and end uses. Thus there is a continuing need for a greater variety of cure catalysts and latent cure accelerators, in order to allow the resin formulator to modify the curing behavior and storage characteristics of cyanate ester-based resin formulations, thereby becoming better able to meet the demands of these industries.
Reinforced composites based on cyanate ester matrix resins have been disclosed in the art. For example, cyanate ester-fiber glass laminates cured with metal salts such as cobalt naphthenate are described in U.S. Pat. No. 4,477,629, and the use of compositions comprising cyanate esters and bismaleimide resins, optionally including curing agents such as imidazoles or amines, as matrix resins with glass fiber is disclosed in U.S. Pat. No. 4,110,364. Although such materials are disclosed to have reduced moisture sensitivity, prepreg based on such compositions lack the latency characteristics desired by the industry. In addition, the presence of cure accelerator residues, and particularly those that are metal-based, tend to reduce the ability of the composite to resist extended exposure to moisture and to other hostile environments.
Latent curing characteristics are particularly beneficial in the manufacture of fiber-reinforced composites. While matrix resin and prepreg may be stored under refrigeration to prevent premature cure prior to use, lay-up of the prepreg when forming complex composite articles often requires lengthy exposure to ambient temperatures. Cyanate ester matrix resin formulations that have rapid curing characteristics may cure prematurely under ambient conditions, leading to loss of tack and resulting in a defective composite. Cyanate ester-based matrix resins formulated without cure accelerators may retain surface tack for sufficient time to complete the lay-up operation, but the resulting article will be difficult to fully cure due to the necessity for heating the article uniformly and throughout its substantial bulk. In addition, when the initial curing step is carried out under pressure in an autoclave, the uncatalyzed resin gels slowly, causing excessive resin flow and a squeezing-out of the resin from within the composite article.
Cyanate ester matrix resin formulations with good latency are needed to provide prepreg that will retain good tack at ambient temperatures during the lay-up operation, and then gel rapidly at moderate temperatures early in the initial heating cycle, thus resisting excessive resin flow and reducing resin loss from the structure. In addition, a cyanate ester matrix resin formulation that attains a high Tg relatively quickly in an accelerated cure is desired by the industry in order to produce a composite article that will attain good dimensional stability rather rapidly and at temperatures lower than would be needed to cure formulations without accelerator. Such articles could then be subjected to an unsupported postcure operation conducted in an ordinary oven, thus reducing the need for costly autoclave process equipment.